The present invention relates to a method and apparatus for cementing and packing off a liner within a well, and more particularly to a one trip liner packer, and still more particularly to a liner packer for packing off a liner within the well.
Typically, in the drilling of a well, a borehole is drilled from the earth's surface to a selected depth and a string of casing is suspended and then cemented in place within the borehole. A drill bit is then passed through the initial cased borehole and is used to drill a smaller diameter borehole to an even greater depth. A smaller diameter casing is then suspended and cemented in place within the new borehole. Generally, this is repeated until a plurality of concentric casings are suspended and cemented within the well to a depth which causes the well to extend through one or more hydrocarbon producing formations.
Oftentimes, rather than suspending a concentric casing from the bottom of the borehole to the surface, a liner is suspended either adjacent the lower end of a previously suspended and cemented casing or from a previously suspended and cemented liner. The liner extends from the previously set casing or liner to the bottom of the new borehole. A liner is casing which is not run to the surface. A liner hanger is used to suspend the liner within the lower end of the previously set casing or liner. Typically, the liner hanger has the ability to receive a tie back tool for connecting the liner with a string of casing which extends from the liner hanger back to the surface. Liners may be used for both land and offshore wells.
A setting tool disposed on the lower end of a work string is releasably connected to the liner hanger which is attached to the top of the liner. The work string lowers the liner hanger and liner into the open borehole extending below the lower end of the previously set casing or liner. The borehole is filled with fluids such as drilling mud which flows around the liner and liner hanger as the liner is run into the borehole. The assembly is run into the well until the liner hanger is adjacent the lower end of the previously set casing or liner and the lower end of the liner is above the bottom of the open borehole. As can be appreciated, it is desirable to have the inside diameter of the liner be as large as possible to allow more space for additional liners to be disposed within the well.
When the liner reaches the desired location relative to the bottom of the open borehole and the previously set casing or liner, the setting tool is actuated to move slips on the liner hanger from a retracted position to an expanded position and into engagement with the previously set casing or liner. Thereafter, when weight is applied to the hanger slips, the slips are set to support the liner.
The liner hanger setting tool may be actuated either hydraulically or mechanically. See U.S. Pat. No. 4,712,614. The setting tool can have a hydraulically operated setting mechanism for the hanger slips or can have a mechanically operated setting mechanism for the setting slips. A hydraulically operated setting mechanism typically employs a hydraulic cylinder which is actuated by pressure in the bore of the work string. In mechanically setting the liner hanger, it is usually necessary to obtain a relative downhole rotation of parts between the setting tool and liner hanger to release the hanger slips. The hanger slips are then one-way acting in that the hanger and liner can be raised or lifted upwardly but a downward motion of the liner sets the slips to support the hanger and liner within the well.
Then to release the hanger, the setting tool is lowered with respect to the liner hanger and rotated to release a running nut on the setting tool from the liner hanger. Cement is then pumped down the flowbore of the work string and liner and up the annulus formed by the liner and open borehole. Before the cement sets, the liner hanger setting tool and work string are removed from the borehole. In the event of a bad cement job, a liner packer and liner packer setting tool are then attached to the work string and lowered back into the borehole. The packer is set utilizing the liner packer setting tool.
Packers for liners are often called liner isolation packers. A typical liner top isolation packer system includes a packer element mounted on a mandrel. A seal nipple is disposed below the mandrel which stings into a tie back receptacle on top of or below the liner hanger. A liner isolation packer is used to seal the liner in the event of a bad cement job. Typically, the liner isolation packer is set down on top of the hanger and the packer is set by a setting tool to form a seal of the annulus between the liner and the previously set casing or liner.
Generally, the deeper a well is drilled, the higher the temperature and pressure which is encountered. Thus, it is desirable to have liners with liner packers which will ensure quality cementing of the liner so as to provide a high safety factor in preventing gas from the formation from migrating up the annulus between the liner and outer casing.
During the cementing operation, drilling mud or fluid in the annulus between the liner and outer casing is displaced by cement as the cement is pumped down the flowbore of the work string. First the drilling mud and then the cement flows around the lower end of the liner and up the annulus. If there is a restriction to flow in the annulus, the flow of the cement slows and a good cementing is not achieved. Any slowing of the cementing in the annulus allows time for the gas in the formation to migrate up the annulus and through the cement to prevent a good cementing job.
Prior art liner top packers restrict the bypass in the annulus at the point of the liner packer. The diameter of the liner packer is just below the drift of the outer casing that the liner is being run through. The increased diameter allows for sufficient back-up for the liner packer to seal properly. However, this also restricts the bypass flow area around the packer causing higher fluid velocities and lower pressures that will either fluid cut the packing element or swab it off entirely. The reduced bypass area also tends to be a stopping location for any solids that may be washed up the well. These solids can packoff at the liner so as to set the liner packer prematurely.
In the cementing operation, the drilling mud is first pumped through the well at a high rate of speed to "clean the well" of any deleterious material. If there is a restriction at the liner packer, the fluid flow may cause the liner packer to set prematurely. If the liner packer sets prematurely, it seals the annulus to fluid flow and the cement can no longer be pumped down hole.
Conventionally, the liner packer has all of its setting mechanism disposed on a mandrel that is located on the outside diameter of the liner, i.e. is located in the annulus formed by the liner and outer casing. The by-pass area around the liner packer for the cement, is formed by the annulus or annular space between the outer casing and liner. Thus, locating the setting mechanism for the liner packer on the outside of the mandrel limits and restricts the annular space for allowing the cement to by-pass the liner packer. Once the by-pass area is set, only a certain volume of drilling mud and cement is allowed to pass around the liner packer at any given time. If the flow rate of the drilling mud and cement is such that the drilling mud and cement cannot bypass the liner packer fast enough, the liner packer becomes a restriction to flow providing a back pressure on the drilling mud and cement.
As an example of the above, if a liner has a diameter of 113/4 inches and the outer casing has a diameter of 133/8 inches, the annular space around the liner packer provides approximately 12.2 square inches of bypass. When the flow of cement encounters the liner packer, the bypass area is reduced to approximately one-half thereby causing the velocity of the fluid flow over the packoff elements of the packer to increase dramatically. This causes the fluid to cut the packing elements and may cause the packing elements to be eroded away. As the fluid flow over the packing elements increases, a low pressure area is created causing the packing elements to expand and get sucked up into the fluid flow. Further, as indicated above, this restriction to flow causes surge pressures downhole because the pumps at the surface are forcing fluids into a fluid filled annular column and the fluid flow is restricted at the liner packer causing the pressure to increase. If this pressure becomes great enough, the drilling fluids or cement may be forced into the formation causing formation damage.
Further, the packing elements of prior art liner packers have not been sufficiently rigid to allow the packing elements to stroke over a port in the packer mandrel since it is necessary for the prior art packing element to fit tightly around the mandrel to ensure sealing engagement. Thus, in prior art packers, any attempt to stroke the packing element over a port causes the edges of the port to tear or damage the interior sealing surface of the packing elements thus preventing them from attaining an adequate sealing engagement upon setting the liner packer.
The present invention overcomes the deficiencies of the prior art.